Leadframe alteration to direct compound flow into package

ABSTRACT

A leadframe comprising a downset formed adjacent to an edge of the leadframe so as to direct the molding compound to flow evenly inside the mold cavity. The downset has an upward slope extending from the edge of the frame and levels off with the rest of the frame at a first transition point. The upward slope facilitates the upward flow of the molding compound entering from a bottom gate. Likewise, the leadframe also directs flow in a top gated mold by reversing the orientation of the leadframe or by forming a reverse downset on the leadframe.

RELATED APPLICATIONS

[0001] This application is a divisional application of U.S. applicationSer. No. 09/489,113, filed Jan. 21, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to packaging of semiconductor integratedcircuits and, more particularly, to a leadframe that directs the moldingcompound to flow evenly into the package during the encapsulationprocess.

[0004] 2. Description of the Related Art

[0005] Semiconductor packaging generally involves incorporatingcompletely fabricated chips, generally referred to as a die, intoprotective packages so that the dies are protected from environmentalcontaminants and handling damage. A common method for packagingintegrated circuits comprises bonding each die to a leadframe andsubsequently encapsulating the die and a portion of the leadframe in amolded epoxy enclosure. Generally, the leadframe has paddles thatreceive the die and also has lead fingers that provide easier contactpoints for external electrical connection to the electronic componentsof the die. Hence, the leadframe provides structural support for the dieand allows the encapsulated die to establish connection with externalstructures such as printed circuit boards.

[0006] More particularly, the leadframe is typically made from a thinsheet of metal and the die paddle that is configured to receive the dieis typically downwardly recessed to the rest of the frame so as toaccommodate for the thickness of the die mounted to the paddle.Furthermore, the leadframe comprises a plurality of leadframe fingersthat are either stamped or etched on the leadframe and extend from anedge of the die paddle to an edge of the leadframe. An inner end of eachleadframe finger is wire bonded to a bonding pad on the die while anouter end of each finger is designed to form contacts with externalstructures so as to establish a plurality of conductive paths betweenthe die and the external structures. The number and configuration ofleadframe fingers vary depending on the particular die design.

[0007] Moreover, an inner section of each leadframe finger and the dieitself are typically encapsulated in a plastic enclosure so that theyare protected from damage and contaminants. During the encapsulationprocess, the die is initially bonded to the leadframe and the die andleadframe structure is placed in a mold cavity wherein the configurationof the cavity defines the shape and size of the resulting protectiveenclosure. Furthermore, a molding compound such as plastic resin isinjected into the cavity so as to complete the formation of a typicallyrectangular shaped enclosure that protects the die and the innersections of the leadframe fingers. Advantageously, the molding operationcan be set up so that multiple dies can be encapsulated in a singleprocess run, thereby providing a cost effective and efficient way ofpackaging integrated circuits.

[0008] However, one disadvantage of the standard encapsulation processis that the mold cavity configuration and positioning of the leadframetherein preclude the molding compound from flowing evenly inside thecavity. This can cause an uneven distribution of resin around the dieand the leadframe resulting in a less effective protective structure. Inparticular, the mold cavity typically comprises a top and a bottom platethat mate to form the cavity. The leadframe is placed in the middle ofthe cavity between the two plates. Furthermore, the mold is typicallydesigned so that the molding compound or resin enters the cavity from anopening or gate formed in one corner of the mold. In a bottom gated moldconfiguration, for instance, the molding compound enters the cavity froma bottom corner and the molding compound fills the bottom portion of thecavity more quickly than it fills the top portion of the cavity. Hence,molding compound entering from a bottom gated mold tends to conglomeratenear the bottom of the cavity because its upward flow path is hinderedby numerous horizontally extended leadframe fingers. Likewise, in a topgated mold configuration, the flow of the molding compound also canconglomerate in the top portion of the cavity as the downward flow pathis obstructed by the horizontally extended leadframe fingers.Disadvantageously, non-uniform compound flow rate in the mold and unevenresin distribution between the top and bottom surface of the leadframeare known to cause defects such as voids, pinholes, and knitlines in thecured plastic enclosure.

[0009] To address this problem, mold design modifications and leadframealterations have been developed in the past in an effort to achieve amore uniform resin flow inside the mold cavity. For example, U.S. Pat.No. 5,965,078 discloses a mold design used in combination withprepackaged molding compound inserts that are inserted in the cavity ofthe mold with the leadframe and die so as to provide a more uniformcompound flow inside the cavity. However, it can be appreciated thatmold modifications with prepackaged inserts are costly to implement andtherefore undesirable in light of the ever increasing demand for costreduction in semiconductor fabrication.

[0010] The prior art also discloses a leadframe having one or moreencapsulate diverters wherein the flow diverters are designed to guideportions of the upper resin flow to a bottom section of the cavity. (Seee.g., U.S. Pat. No. 5,926,695). However, the flow diverter as taught byprior art is applicable only for molds that are top gated wherein theresin is introduced from a top corner of the mold. Furthermore, thediverter is positioned away from the gate and therefore unable to directthe flow of the compound at the point where the resin first enters thecavity. In fact, the diverter is designed to guide the resin flow onlyafter the resin has already reached the paddle area where air pocketsthat are known to cause voids and pinholes are likely to have alreadydeveloped.

[0011] Moreover, the leadframe diverters as suggested by prior art areessentially unsupported flaps that can be easily damaged during theencapsulation process. In particular, it can be appreciated that anunsupported thin layer of metal bent at an angle can be deformed orbroken off by the flow of the compound injected into the mold cavity.Furthermore, leadframes with the flow diverters as suggested by priorart are time consuming to manufacture as it requires additionalprocessing steps to ensure that the diverters bend at a particular anglerelative to the leadframe.

[0012] Further, the flow diverter disclosed in the patent also requiresthat space on the die paddle be occupied by the opening to permit theresin to flow through. It will be appreciated that given the everincreasing need for greater density devices, the space on the diepaddles and on the lead frame is becoming increasingly limited. Hence,for many high density leadframe designs, forming openings simply forresin flow is impractical and inefficient.

[0013] Hence, from the foregoing, it will be appreciated that there is aneed for a leadframe wherein the leadframe directs the flow of themolding compound to flow more evenly inside the mold cavity during theencapsulation process. To this end, there is particular need for analtered leadframe that is able to guide the flow of the compound as thecompound enters the cavity so as to achieve a uniform resin flow ratefrom the outset and thereby minimize defects such as pinholes, voids andknitlines in the molded enclosure. Furthermore, it is desirable that theleadframe is effective in directing compound flow in both top and bottomgated molds. Furthermore, it is also desirable that such leadframealterations can be implemented quickly and cost effectively.

SUMMARY OF THE INVENTION

[0014] The aforementioned needs are satisfied by the leadframe of thepresent invention. In one aspect, the present invention discloses aleadframe wherein the leadframe comprises a plurality of lead fingersand a die paddle adapted to receive the die and a second offsetpositioned adjacent an edge of the leadframe. In this aspect, the secondoffset is positioned so as to increase the pressure differential betweenthe first and the second surfaces of the leadframe so as to increase theflow of compound from the first surfaces to the second surface via thespaces between the plurality of lead fingers. In this way, the flow ofcompound can be increased without requiring limited space on theleadframe be occupied by flow openings and the like.

[0015] In one embodiment, the offset comprises a downset comprising adownward indentation formed on a top surface of the leadframe wherein atop surface of the indentation slopes upwardly towards the paddle untilit becomes level with a general plane defined by the top surface of theleadframe.

[0016] In one embodiment, the leadframe further comprises a tie barwherein an outer end of the tie bar is generally triangular andcomprises the offset. Preferably, in an encapsulation process, theleadframe is placed inside a mold cavity in a manner such that theoffset is positioned adjacent a gate. Preferably, the offset is designedto receive a portion of the molding compound entering from the gate andguide the portion to flow toward the second surface of the leadframe. Inparticular, the offset produces a localized increase in the pressure ofthe compound flowing from the gate. This localized increase in pressureresults in a greater pressure differential between the side of theleadframe adjacent the gate and the side opposite the gate therebyincreasing the flow from the side adjacent the gate to the side oppositethe gate.

[0017] Advantageously, the altered leadframe of the preferred embodimentprovides an offset comprising a sloped surface strategically positionedto facilitate the flow of the molding compound as the compound entersthe cavity from a gate. In particular, the altered leadframe splits theflow of the compound as the compound enters from a gate so as to ensurea more even distribution of the compound between the top and bottom ofthe leadframe.

[0018] From the foregoing, it will be appreciated that the aspects ofthe present invention provide a new, altered leadframe that is designedto direct molding compound to flow more evenly inside a mold cavity in acost-effective manner. Moreover, the altered leadframe can be adapted todirect compound flow inside mold cavities that are either top or bottomgated. These and other objects and advantages of the present inventionwill become more apparent from the following description taken inconjunction with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a top down view of a leadframe of the preferredembodiment;

[0020]FIG. 2 is a partial schematic side view of a conventionalleadframe positioned inside a mold cavity and the flow direction of themolding compound;

[0021]FIG. 3 is a partial schematic side view of the leadframe of thepreferred embodiment positioned inside a molding cavity and the flowdirection of the molding compound.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] Reference will now be made to the drawings wherein like numeralsrefer to like parts throughout. As will be described hereinbelow, theleadframe of the preferred embodiment provides a leadframe thateffectively directs the molding compound to flow evenly around the dieand leadframe during encapsulation so as to reduce voids and pinholes inthe encapsulated lead frame that typically result from uneven resin flowinside the mold cavity.

[0023]FIG. 1 illustrates a general top down view of a leadframe 100 ofthe preferred embodiment. As is shown in FIG. 1, the leadframe 100 isgenerally rectangular in shape and is typically made from a thin sheetof metal such as a copper or nickel alloy. As will be described ingreater detail below, the leadframe 100 is designed to providestructural support for a die or chip and also facilitate electricalinterconnection to the components formed on the die. In the typicalpackaging process, the die is encapsulated in a plastic enclosure.Furthermore, the leadframe 100 is also configured to provide a pluralityof conductive paths between the encapsulated die and external structuressuch as printed circuit boards.

[0024] As is illustrated in FIG. 1, the leadframe 100 comprises a diepaddle 102 that is a generally rectangular region located in the centerof the leadframe 100 and is formed by a well known stamping or etchingprocess. In particular, the paddle 102 is configured to seat a die 104and defines a seating surface 105 that is generally larger than thedimensions of the die 104. Preferably, the paddle 102 is also recessedor downset relative to the rest of the leadframe 100 so as toaccommodate the thickness of the die 104 that is bonded to the paddle102.

[0025] Furthermore, as is also shown in FIG. 1, the leadframe 100comprises a plurality of leadframe fingers 106 that are generallyelongated and extend from an edge 110 of the die paddle 102 to an edge112 of the leadframe 100. Preferably, the leadframe fingers 106 are alsoformed by stamping or etching the leadframe 100 using a method wellknown in the art. Furthermore, an inner end 114 of each finger 106 istypically wired bonded to the die 104, while an outer end 116 of eachfinger 106 is adapted to connect with external structures such as aprinted circuit board. The leadframe fingers 106 thus effectively serveas a plurality of conductive paths between the die 104 and externalstructures.

[0026] As FIG. 1 further illustrates, each leadframe finger 106 extendsoutwardly from the edge 110 of the paddle 102 to a dambar 120.Preferably, the dambar 120 has an elongated shape and is formed by thesame etching or stamping process that is used to create the paddle 102and the fingers 106 on the leadframe 100. As is also shown in FIG. 1,each dambar 120 spans in a direction generally perpendicular to theleadframe fingers 106 and is positioned approximately halfway betweenthe edge 110 of the paddle 102 and the edge 112 of the leadframe 100. Inparticular, the dambar 120 is connected to each of the leadframe fingers106 so as to provide rigidity and structural support for the fingers 106during the encapsulation process during which the fingers aresusceptible to damage. Furthermore, the dambars 120 also block andprevent the molding compound from streaking onto an outer section 107 ofthe fingers 106 during encapsulation. Subsequent to encapsulation, thedambars 112 are severed from the fingers 106 in a singulation processthat is well known in the art. In particular, the singulation processseparates the fingers 106 from the dambar 112 so that the fingers 106become disconnected from the dambar 112 and from each other.

[0027] As is also shown in FIG. 1, a plurality of tie bars 122 a, 122 bextend from the edge 110 of the paddle 102 to the edge 112 of theleadframe 100. Preferably, the tie bars 122 a. 122 b are generallyelongated and are formed using a well known stamping or etching process.As shown in FIG. 1, the tie bars 122 a, 122 b ensure that theisland-like paddle region 102 located in the center of the leadframe 100is structurally connected to the rest of the frame 100. In particular,an inner end 115 a, 115 b of each tie bar 122 a, 122 b is connected tothe edge 110 of the paddle 102 while an outer end 117 a, 117 b isattached to the edge 112 of the leadframe 100.

[0028] Furthermore, as shown in FIG. 1, a plurality of dashed lines 126define a generally rectangular inner region of the leadframe 100 whereinthe region comprises the die paddle 102 and an inner section 130 of theleadframe fingers 106 extending between the paddle 102 and the dambar120. Preferably, once the die 104 is mounted to the paddle 102, theinner region of the leadframe 100 is encapsulated in an enclosure in amanner to be described in greater detail below. Preferably, during theencapsulation process, the inner region of the leadframe 100 is placedinside a mold cavity wherein molding compound is introduced into thecavity from an area adjacent the outer end 117 a of one of the tie bars122 a.

[0029] In particular, a plurality of arrows 132 adjacent the outer end117 a of the tie bar 122 a as shown in FIG. 1 indicate the generaldirection in which the molding compound flows into the cavity during theencapsulation process of a preferred embodiment. The arrows 132 show themolding compound entering the cavity from a gate 134 that is locatedadjacent the outer end 117 a of the tie bar 122 a. Preferably, themolding compound flows in a generally horizontal direction from a firstgate, which, in this embodiment, is a bottom lower gate 134 of thecavity and fills the entire cavity so that the top and bottom sides ofthe leadframe are completely covered. Disadvantageously, however, themold configuration and positioning of the leadframe inside the moldcavity often cause the resin to flow unevenly around the leadframe. In abottom gated mold, for instance, the molding compound is shown to favorthe bottom of the cavity because the upward resin flow is often hinderedby the numerous horizontally extended leadframe fingers 106. Likewise,in a top gated mold wherein the compound enters from a top corner of thecavity, the molding compound is shown to gather more around the top ofthe leadframe as the downward resin flow is blocked by the horizontallyextended fingers 106.

[0030] In the preferred embodiment, the leadframe 100 is altered so thatthe leadframe is able to direct the compound flow once the compoundenters the cavity so as to more evenly split the compound between thetop and bottom of the leadframe. In particular, the outer end 117 a ofthe tie bar 122 a positioned adjacent the gate 134 is offset or downsetrelative to the horizontally extended leadframe fingers 106 so as tofacilitate the upward flow of the compound. As will be described ingreater detail below, an indentation 125 is formed on a top surface 123a of the outer end 117 a of the tie bar 122 a. Preferably, theindentation 125 is comprises a first and a second sidewall forming agenerally v-shaped groove. Each sidewall slopes upwardly until itbecomes level with the general plane defined by the top surface of theleadframe 100.

[0031]FIG. 2 provides a partial side view of a conventional prior artleadframe 200 positioned inside a mold cavity 208 during anencapsulation process. As shown in FIG. 2, the conventional leadframe200 comprises a die paddle 202 that is downset and bonded to a die 204.Furthermore, a plurality of leadframe fingers 206 extend outwardly fromthe paddle 202 to an edge 212 of the leadframe 200. As is alsoillustrated in FIG. 2, the die 204 and an inner region 224 of theleadframe 200 are placed inside the mold cavity 208 defined by a top andbottom plate 137 a, 137 b. In particular, the mold cavity 208 defines agenerally rectangular enclosure wherein the inner region 224 of theleadframe 200 is positioned horizontally across approximately the middleof the cavity 208. Furthermore, it is generally known that a moldtypically has an injection port or a gate that is located on either thetop or bottom corner of the cavity that comprises an opening whichallows molding compound to enter the cavity. As is shown in FIG. 2, theillustrated mold 208 has a gate 234 that is formed on the bottom plate137 b, however it is appreciated that the gate can also be positioned onthe top plate 137 a.

[0032] As is also shown in FIG. 2, a plurality of arrows 139 demonstratethe path of the compound flow once the compound enters the mold cavity208 through the bottom gate 234. The arrows 139 illustrate an unevencompound flow wherein more compound is shown to reach a bottom region138 b of the cavity and conglomerate around a bottom surface 201 b ofthe leadframe 200. In particular, the upward flow path of the compoundis obstructed by the horizontally extended leadframe fingers 206,therefore less compound is able to reach the top of the frame 200.Consequently, an uneven flow pattern as illustrated by the arrows 139 isknown to cause defects such as pinholes, voids, and knitlines in theresulting plastic enclosure.

[0033] To minimize the occurrence of such defects, the leadframe 100 ofthe preferred embodiment is configured to direct resin to flow moreevenly inside the mold cavity so as to minimize the formation of airpockets during encapsulation. With reference to FIG. 3, the leadframe100 of the preferred embodiment is shown to be positioned inside aconventional bottom gated mold cavity as described in FIG. 2. Asdescribed in detail above, the leadframe 100 of the preferred embodimentgenerally comprises the paddle 102 bonded to the die 104 and numerousleadframe fingers 106 extending outwardly from the paddle 102.Furthermore, as illustrated in FIG. 3, the paddle 102 of the leadframe100 comprises a first offset which, in this embodiment, is a firstdownset 105 that is a generally rectangular indentation formed by aknown punch press operation and configured to accommodate the thicknessof the die 104 that is seated in the paddle 102.

[0034] As is also shown in FIG. 3, the leadframe 100 of the preferredembodiment also has a second offset comprising a second downset 150 thatis a generally v-shaped indentation 152 formed on the top surface 123 aof the outer end 117 a of the first tie bar 122 a. Preferably, theindentation 152 is formed in the same punch press operation as that usedto form the first downset 105. Preferably, the indentation 152 has afirst and a second side wall 159 a, 159 b and each side wall slopesupwardly until it becomes level with a top surface 158 of the leadframefingers 106. Furthermore, the length of the indentation extends from atop surface of the first side wall 159 a to a top surface of the secondsidewall 159 b and the depth extends from a bottom 161 of theindentation to the top surface 158 of the leadframe fingers. As isillustrated in FIGS. 1 and 3, the indentation 152 comprising the seconddownset 150 is located on the end 117 a of the first tie bar 122 a so asto be immediately adjacent the compound gate 134. The indentation 152preferably extends into the flow path 132 of the compound that is beinginjected into the mold 208 during the injection molding process. Thisresults in the pressure of the compound at the injection port 234 beinglocally increased.

[0035] Hence, there is a greater pressure differential between thecompound located adjacent the bottom surface 171 of the leadframe 100than the upper surface 173 of the leadframe. As a result of this greaterpressure differential, more of the compound that is entering adjacentthe bottom surface 171 is induced to flow to the upper surface 173through the openings between the lead fingers 106. Hence, as a result ofthis greater flow of compound, the formations of pinholes, knitlines andvoids in the compound is reduced due to the more even distribution ofinjection resin or compound within the mold 208.

[0036] Furthermore, it can be appreciated that the above describedaltered leadframe can be adapted to direct the flow of compound inside atop gated mold. In particular, the leadframe 100 can be placed in anupside down orientation inside a top gated mold so that the seconddownset 150 will guide the compound entering from the top of the cavityto flow downwardly into the bottom plate. In particular, the leadframe100 can be positioned upside down wherein a top surface 101 of thepaddle 102 faces the bottom plate of the mold. Preferably, when theleadframe 100 is placed in an upside down orientation, the seconddownset 150 will facilitate the compound entering from the top gate toflow downwardly toward the bottom of the cavity. Preferably, theleadframe 100 directs the compound flow so as to more evenly direct thecompound between the top and bottom of the plate and therefore minimizethe formation of air pockets that cause defects such as pinholes, voids,and knitlines in the resulting enclosure.

[0037] Alternatively, the second offset 150 can also be formed in anupside down configuration wherein the indentation is formed on a bottomsurface 158 of the first tie bar 122 a. In one embodiment, theindentation protrudes upwardly so as to direct a portion of the compoundentering from a top gate to flow downwardly to the bottom of the cavity.Preferably, the configuration of the second offset 150 is identical asthe second downset 150 when the leadframe 100 is positioned upside downinside the mold cavity.

[0038] Advantageously, the alteration to the leadframe 100 is not costlyto implement as it can be achieved with relative minor changes to apunch press die. Therefore, the present invention provides an alteredleadframe wherein the frame allows the molding compound to flow evenlyduring the encapsulate molding process. The alteration is relativelyinexpensive to implement as it involves an extra press operation thatuses existing equipment and tools with minor modifications. Unlike knownleadframe modifications, the altered leadframe effectively distributesthe flow of the compound from the point when the compound first entersthe mold so as to provide a more even flow from the outset.

[0039] Although the foregoing description of the preferred embodiment ofthe present invention has shown, described and pointed out thefundamental novel features of the invention, it will be understood thatvarious omissions, substitutions, and changes in the form of the detailof the apparatus as illustrated as well as the uses thereof, may be madeby those skilled in the art, without departing from the spirit of theinvention. Consequently, the scope of the present invention should notbe limited to the foregoing discussions, but should be defined by theappended claims.

What is claimed is:
 1. A method of encapsulating a leadframe having adie paddle adapted to receive a die and a plurality of lead fingers, themethod comprising: forming an offset in the leadframe; positioning theleadframe in a mold such that the offset is immediately adjacent aninjection port through which encapsulating compound is to be injectedinto the mold; injecting encapsulating compound into the mold such thatthe injected encapsulating compound interacts with the offset therebyincreases the pressure differential between a first and a second surfaceof the leadframe so as to increase the amount of encapsulating compoundflowing between the first and the second surface of the leadframe viathe spaces between the lead fingers.
 2. The method of claim 1 , whereinforming an offset in the leadframe comprises forming an indentation inthe leadframe so that the indentation is offset from a plane of theleadframe defined by a surface of a plurality of lead fingers.
 3. Themethod of claim 2 , wherein forming an indentation comprises forming adownset and wherein positioning the leadframe in a mold comprisespositioning a leadframe in a mold having a bottom gated injection port.4. The method of claim 1 , wherein the die paddle and the offset areformed in a same punch press operation.
 5. The method of claim 2 ,wherein forming an indentation comprises forming an indentation having aheight of approximately 0.009 inches and a length of approximately 0.087inches.
 6. The method of claim 5 wherein forming an indention comprisesforming an indentation in an outer end of a tie bar of the lead frame.